Specific protection of the binding sites of D-Ala2-D-Leu5-enkephalin (delta-receptors) and dihydromorphine (mu-receptors).

Phenoxybenzamine causes a long-lasting inactivation of the opiate receptors of the mu- and delta-type in homogenates of guinea-pig brain. The effect is selectively prevented when, before exposure to phenoxybenzamine, the homogenate is pre-incubated with ligands of high affinity for either of the two binding sites, i.e. dihydromorphine for the mu-receptor and Tyr-D-Ala-Gly-Phe-D-Leu for the delta-receptor. In contrast, Tyr-D-Ala-Gly-Phe-L-Leu amide, which has high affinities for both binding sites, protects both receptor sites.